Magnetic resonance imaging (MRI) is well known for its high resolution, unlimited tissue penetration in depth and absence of ionizing radiation that exceeds other biomedical imaging modality. Generally, contrast agents (CAs) for MRI are classified into T2-weighted (r2/r1 ratio>>1) and T1-weighted (r2/r1 ratio close to 1) types.
For T2 CAs, superparamagnetic iron oxide nanoparticles (NPs) with high r2 relaxivities have been used in clinical hepatic tumor detection. However. T2-weighted imaging that produces negative contrast usually raises confusion with the signals from hemorrhage because both of which exhibit negative signals.
On the other hand, the T1-weighted counterpart, dubbed as the positive contrast, is subject to less artefacts and has higher signal intensity of the vascularized tissues, thereby providing advantageous alternatives. However, many clinically available T1 CAs, such as Gd-based (e.g. Gd-complexes, Gd2O3 NPs) and Mn-based (e.g. MnO, Mn3O4 NPs) CAs, are potentially hazardous or even life-threatening.
Accordingly, it is urgent to develop new contrast agents with both superior contrast and high biocompatibility.